This disclosure relates generally to the field of downhole measurements. More specifically, the disclosure relates to using pulsed neutron gamma-ray spectroscopy to measure formation water salinity from within a borehole.
Hydrocarbons are found in porous or fractured rock and are typically accompanied by water beneath. Hydrocarbon and water are also found together in their undisturbed state and particularly after disturbance by hydrocarbon production operations. Subsurface water at the depths one may find hydrocarbons typically has a high concentration of dissolved salt.
In the exploration for hydrocarbons and their subsequent production it is the practice to make measurements of subsurface rock and fluid properties from within boreholes. The instruments used to perform these measurements are varied and make use of different physics principles. Some of the instruments are sensitive to the presence of salt in the water and others are not. For those that are sensitive to salt, it may be important to account for the amount of salt in the water, the water salinity, or accommodate some amount of error in the measurement of the rock or fluid properties due to an incorrect accounting of the salinity.
An example of a borehole measurement instrument that is sensitive to the presence of salt in water is the borehole resistivity sonde, which measures the electrical resistivity of the fluid in the pores of the rock. The water saturation, Sw, which is the fraction by volume of water within the pores, is inferred from the resistivity measurement. Salt in the water determines water resistivity; low water resistivity results from high salinity and high water resistivity results from low to zero salinity. Interpreting the measurement of the borehole resistivity sonde to obtain water saturation often requires knowledge of the water resistivity. Hence the accuracy of the reported water and hydrocarbon saturations are dependent on the accuracy of the estimated or measured formation water salinity. An example of inaccurate water saturation measured by a resistivity sonde in conditions of unknown water salinity is given in Eyvazzadeh, R. Y., O. Kelder, A. A. Hajari, S. M. Ma, and A. M. Behair, “Modern Carbon/Oxygen Logging Methodologies: Comparing Hydrocarbon Saturation Determination Techniques”, SPE 90339, September, 2004.
Values for the salinity of the formation water surrounding a borehole are typically based on one or several fluid samples taken within that borehole, a single sample of produced fluids from that borehole taken at the surface, or a measurement taken at a different well within the same reservoir. These practices are fraught with many opportunities for error in the salinity.
The questionable estimates of salinity are further compromised by the widespread practice in mature reservoirs of injecting water from various sources into the reservoir in order to maintain or restore reservoir pressure and to displace oil in the direction of producing wells. The injected water may be salt-free (fresh) surface water, low-salinity sea water, high-salinity produced water from the same or neighboring reservoirs, or combinations of these. The consequence of water injection is formation water salinity that is changing both in time and space. The salinity will be different from well to well, within the same well, and will change over time. This condition is referred to as mixed salinity. Mixed salinity is a significant source of error for interpretation of the data from borehole measurement sondes that are sensitive to the presence of salt in the formation water.
Certain conventional methods for obtaining formation water salinity measurements exist, each with certain disadvantages and weaknesses. One method is the formation tester sample method. In this technique, a rubber pad is pressed against the wall of a small section of an uncased borehole at a particular depth. The rubber pad forms a rough seal against the formation, ideally a formation with a coating of smeared drilling mud known as mud cake. A small aperture at the center of the pad allows access from the formation to a pump and sample chamber within the sonde. Formation fluid is pumped through the sonde for some minutes to allow removal of borehole fluids from the near borehole formation and allow access to undisturbed formation fluid. After this cleanup period, access is given to the sample chamber via a valve and a sample of the formation fluid is collected for later transport to the surface where a laboratory analysis is performed.
This method is able to determine accurate formation water salinity, and is routinely used during the early life of a well when there is no casing in place. Samples are not likely to be taken once the well has been cased (though it is possible with a cased-hole formation tester), the well is in production, and the time-dependent effects of mixed salinity due to water injection become apparent. The method is usually not applied to producing wells because of the large outside diameter of the sonde and the requirement of a mud cake to seal between the formation and the rubber pad. In addition, it is usually an expensive and time consuming operation that may require a workover rig. When it is done, a very limited number of samples are taken at depths judged to be of high importance.
A second method is the bottom hole sample method. This technique is less complex than the formation tester sample in that it does not have a rubber pad forming a seal with the formation. It comprises a sample chamber with valve that is lowered into the borehole. Any sample collected is of the borehole fluid resulting from production of the well, and is not a true formation fluid sample. Borehole fluid within a producing well can be a mixture of fluids from different layers of the formation. The effect of mixing can be resolved with the additional measurement of fluid flow rate at the same depths as the samples were taken or continuously with depth.
This technique has the advantages of lower cost than the formation tester sample and access to producing wells. Its disadvantages are a larger error in the formation water salinity due to mixing and having access to formation fluids only at the depths where perforations in the casing exist.
A third method is the wellhead sample method. This technique, which involves taking samples of fluid at the head of a producing well, is the least complex and lowest cost measurement. It is also the least representative of the formation water salinity as it is a mixture of the fluids from all producing layers or perforated zones.
A fourth method is to obtain salinity information from a nearby well. In some cases, no measurement of formation water salinity is made for a particular well and a value, based on information from nearby wells, judged to be representative of the reservoir or portion of the reservoir is used for interpretation purposes. This method may provide qualitative information of formation water salinity and can have significant accuracy issues depending on the circumstances.
From the perspective of providing accurate input for the interpretation of data from borehole measurements that are sensitive to the presence of salt, all of these conventional methods suffer from a sparseness of data. A few, or one, or even no measurements of salinity are taken as representative of entire sections of a borehole, when in reality there may be gradients in the salinity over given sections of the borehole and from well to well, and there may be changes with time.
Accordingly, there is a need in the art for methods and systems for obtaining formation water salinity measurements from within a borehole that overcome one or more of the deficiencies that exist with conventional methods.